Supernova Relic Neutrinos (SRN) are a diffuse neutrino signal from all past supernovaethat has never been detected.

Motivation SRN measurement enables us to investigate the history of past Supernovae. The SRN flux is related to the supernova rate in galaxies and the cosmic star formation history

Predicted Predicted SRN fluxSRN flux

Expected number SRN events in SK0.8 -5.0 events/year/22.5kton

(10-30MeV)

0.5 -2.5 events/year/22.5kton(16-30MeV)

0.3 -1.9 events/year/22.5kton(18-30MeV)

Ando et al (2005) (LMA)R. A. Malaney (1997)Kaplinghat et al (2000)Hartmann, Woosley (1997)Totani et all (1996) (constant SN rate)

SK-ISK-IDATADATA

Atmospheric e (dot dashed)

Invisible -e decay (dashed)

(1496 days)

OLDOLD 90% C.L. Flux limit: 90% C.L. Flux limit: SK-I : < 1.2 /cm2 /sec

Two Irreducible backgroundsTwo Irreducible backgrounds:1) Atmospheric νe cc interactions

2) Decay of sub-Cherenkov ‘invisible μ’s’ from atmospheric νμ interactions

From 2003 published resultFrom 2003 published result:SK-I result:

M. Malek, et al, Phys. Rev. Lett. 90, 061101 (2003)

This study used:binned χ2 limit extraction18 MeV lower energy thresholdSK-I data only0th order inverse beta cross sectiontwo irreducible backgroundsThese things have now been improved!

+ 90% c.l. relic

both backgrounds (solid)

solar e recoil energy (total) (MeV)

energy resolution at: 16 MeV 18 MeV

7Be

16 18

8B

pep

hep

pp Nuclear Spallation from cosmic ’sSolar neutrinos

Radioactive backgroundsCosmic ray muons, decay electrons

Pions from neutrino interactionsElectronics effects

• many backgrounds, cuts• solar ’s and spallation: largest at low energy, set energy threshold• dominant background is spallation: spa-cut has largest inefficiency• crude solar and spallation cuts in published analysis: improvement needed for lower E threshold

Reducible Backgrounds

Spallation and Solar CutsSpallation and Solar Cuts11Be

11Li12N14B

energy energy resolutionresolution

8B

9Li

8Li

12B13B13O

12Be

12C

8He9C

15C

16N16 MeV

18 MeV

New threshold 18 16 MeV!

Lowering threshold < 16 MeV too difficult due to “wall” of spallation products with long half-lives that enter sample

• SPALLATION is cut using correlation to cosmic ray muons• Original cut used 2-D spatial correlation, time and charge• New method allows 3-D spatial correlation, muon categorization• Stricter cut < 18 MeV

• SOLAR events are cut by correlation to solar direction• New technique estimates multiple scattering, which dominates angular resolution• New cut is optimized in 1 MeV bins using MC, better reduction

16-18 MeV N/A 23% N/A 18% 18-20 MeV 7% 9% 36% 9% 20-24 MeV 7% 0% 36% 9%24-34 MeV 7% 0% 36% 0%

Energy range 2003 cut new cut

Solar and Spallation cut inefficiency

SOLAR CUT SPALLATION CUT2003 cut new cut

Total signal inefficiency: SK-I (1497 days) SK-II (794d) SK-III (562d) NEW (now) 22% (16-90) 31% 23%OLD (2003) 48% (18-90) N/A N/A

(now more data included!)

Atmospheric backgroundνμ CCνe CCμ/πNC elastic

E of background (MeV)

20032003: two channels:νμ CC spectrum modeled by decay electrons from cosmic ray ’s νe CC spectrum from MC

NowNow: four channels: νμ CC νe CCNC elastic required by lower E threshold; spectrum from MCμ/π prod.: reduced by cuts; helps constrain NC in signal fit

SK-I/III combined final data sample

Cherenkov angle distribution degrees

νe CCμ/π NC elastic

low region(μ/π)

isotropic region(NC elastic)

signal region(relic //e)

MC (without contribution)

low region(μ/π)

signal region(relic //e)

isotropic region(NC elastic)

e e+p

n (invisible)

Signal region

42o

μ, π

Low angle events

25-45o

Isotropic region

N

reconstructedangle near 90o

20-38 degrees 38-50 degrees 78-90 degrees E (MeV)

SK-I/III dataνμ CCνe CCNC elasticμ/π > C. thr.all backgroundrelic

20032003: binned χ2 fit to center region, two background channels

NowNow: simultaneous unbinned maximum likelihood fit, four background channels,three Cherenkov angle regions.Each channel has freefloating normalization

Combined Fit

combined 90% c.l.: < 5.1 ev / yr / 22.5 ktons interacting < 2.7 /cm2/s (>16 MeV) < 1.9 /cm2/s (scaled to >18 MeV)

combined90% c.l.

ev/yr interacting in 22.5 ktons

logL

ikel

ihoo

d

SK-I/II/IIIcombinedlikelihood

Comparison to Published /cm2/s >18 MeV

Published limit 1.2

cross section update to Strumia-Vissani

1.2 1.4

Gaussian statistics Poissonian statistics in fit

1.4 1.9

New SK-I Analysis:ETHRESH 18 16 MeVε = 52% 78 %(small statistical correlation in samples)improved fitting method takes into account NC

1.91.6

New SK-I/II/III combined fit 1.6 1.9

BACKUP

Period Live time # ID PMTs / % coverage Comment

SK-I 1497 days 11146 / 40% Experiment start

SK-II 793 days 5182 / 19 % After accident

SK-III 562 days 11129/ 40% After repair

SK-IV running now 11129/ 40% New electronics

Super-Kamiokande (Super-Kamiokande (SKSK))

SK is 50 kton water Cherenkov detector in the Kamioka mine, Japan (2700 m.w.e). The data is divided into segments: SK-I, II, III, and IV.

Electron energy [MeV]

10

0.1

10-3

10-5

10-7

SK E

vent

Rat

e [/

year

/M

eV]

0

10 20 30 40 50

νe+ 16O 16N + e+ν

e + 16O 16F + e -νe + e ν

e + e -

νe+ p e+ + n The main interaction mode for SRN’s in SK is charged current quasi-elastic interaction (inverse decay)

• 4 variable likelihood cut• The 4 variables:

– dlLongitudinal

– dt– dlTransverse

– QPeak

• Use new, better μ fitters• Tuned for each muon type (i.e.

single, multiple, stopping μ)• Improvements allow lowering of

energy threshold to 16 MeV!

distance along muon track (50 cm bins)

Spallation CutSpallation Cut

QPeak = sumof charge inwindowspallationexpectedhere

New Cut: 16 < E < 18 MeV: 18.2% signal inefficiency 18 < E < 24 MeV: 9.2% signal inefficiencyOld cut (likelihood + 150 ms hard cut) 18 < E < 34: 36% signal inefficiency

μ entry point

μ track

dlTransverse

where peakof DE/DX plotoccurs

dlLongitudinal

dE/dx Plot

Relic CandidateOLD likelihood

NEW!

Effwall cutEffwall cut

Energy (MeV)

oldnew

Some ray events originating from outside of fiducial volume have possibility of being reconstructed within fiducial volume of SK. In order to remove these events, we applied effwall cut which uses travel distance from tank wall.

Signal Inefficiency:: Old: 7% New: 2.5%

Signal Inefficiency:: Old: 7% New: 2.5%

reconstructed event vertexreconstructed

event direction

Effwall

Inner detector wall

Final Backgrounds (after all relic cuts)

E (MeV)

These 3 can be modeled as a combination of other backgrounds, and thus aren’t considered separately

CC Backgroundsinvisible μ decay eνe CCμ > C. threshold μ/π

νμ CCνe CCμ/πNC elastic

Combined Fit

combined 90% c.l.: = 5.1 ev / yr / 22.5 ktons interacting = 2.7 /cm2/s (>16 MeV) = 1.9 /cm2/s (scaled to >18 MeV)

combined90% c.l.

ev/yr interacting in 22.5 ktons

logL

ikel

ihoo

d

SK-I/II/IIIcombinedlikelihood

SK-I (~1500 days)SK-II (~790 days)SK-III (~560 days)combined

SK-I

best fit is negativefit shown is 0 relic contribution

20-38 degrees 38-50 degrees 78-90 degrees

E (MeV)

SK-Idataνμ CCνe CCNC elasticμ/π > C. thr.all backgroundrelicSK-I only 90% c.l. limit:

< 2.4 /cm2/s (>16 MeV)< 1.6 /cm2/s (scaled to >18 MeV)

SK-IIBest fit (shown):3.5 ev/yr interacting

20-38 degrees 38-50 degrees 78-90 degrees

E (MeV)

SK-II only 90% c.l. limit:< 7.4 /cm2/s (>16 MeV)< 5.2 /cm2/s (scaled to >18 MeV)

dataνμ CCνe CCNC elasticμ/π > C. thr.all backgroundrelic

SK-IIIBest fit (shown) :6.5 ev/yr interacting

20-38 degrees 38-50 degrees 78-90 degreesE (MeV)

SK-III only 90% c.l. limit:< 8.1 /cm2/s (>16 MeV)< 5.7 /cm2/s (scaled to >18 MeV)

dataνμ CCνe CCNC elasticμ/π > C. thr.all backgroundrelic

Systematics: Inefficiency

19

• Define:– r = # relic events we see in data– R = # relic events actually occurring in detector– ε = efficiency (SK-I/II/III dependent)– assume ε follows a probability distribution P(ε)– assume P(ε) is shaped like Gaussian w/ width

σineff

– then we alter likelihood:

then the 90% c.l. limit R90 is such that

dPRLRL )()()('1

0

σineff

SK-I: 3.5%SK-II: 4.5% SK-III: 3.1%

Cuts: efficiencies and sys errors

20

• SK-I:effwall : 98% (0.5%)C. angle: 95% (0.4%)pion like: 98% (0.2%)spall+solar: 89% (1%)2-peak, 2-ring: >99%Correlation cut: 99% (0.3%)1st reduction: 99% (2%)

(includes: electronic noise cuts, 50 us cut)

Total: 78 %

• SK-II95% (0.3%)88% (0.3%)97% (0.5%)87% (1.4%)>99%99% (0.3%)99% (2%)

• SK-III96% (0.3%)94% (0.3%)98% (0.5%)89% (1%)>99%99% (0.3%)99% (2%)

69% 77%